Mode-locked lasers including a solid-state gain-medium provide very short pulses at relatively high pulse-repetition rates. Typical mode-locked lasers operate at a pulse repetition frequency of between about 50 and 150 MHz megahertz (MHz) with 80 MHz being typical. Depending on the gain-medium, pulses can have a FWHM duration of about 100 picoseconds or less. Most commercially available mode-locked lasers employ a solid-state gain-medium with a relatively broad gain-bandwidth. The most common solid-state gain-medium is titanium-doped aluminum oxide (Ti:Sapphire or Ti:Al2O3) which can provide gain with a limited range of tunability in a fundamental wavelength range between about 700 nanometers and 900 nanometers. The wavelength range of such lasers can be extended by frequency conversion of the output radiation having the fundamental wavelength.
A common application of mode-locked laser pulses is for fluorophore excitation in multi-photon microscopy. This application would benefit from the availability of mode-locked pulses the wavelength of which could be tailored for specific fluorophores.
OPS-lasers include a multilayer semiconductor surface-emitting gain-structure having active or quantum-well (Q-W) layers spaced apart by spacer layers. The output wavelength of such lasers can be “tailored” to a particular value by selecting an appropriate composition of the semiconductor material of the active layers. This can provide, in theory at least, fundamental wavelengths from the ultraviolet spectral region to the mid-infrared region of the electromagnetic spectrum. A characteristic of semiconductor gain-media is that the excited-state lifetime of such gain-media is relatively very short, for example, about 10 nanoseconds (ns) or less, compared with one millisecond or more for a solid-state gain-medium. It is for this reason that mode-locking in semiconductor lasers has only been achieved with very short resonators, at correspondingly very high pulse repetition frequency (PRF), for example, a few gigahertz (GHz). This provides that multiple round trips in the resonator can be achieved within the excited-state lifetime of the semiconductor gain-medium. Unfortunately, the pulse energy achievable at such a high PRF is too low and the PRF is too high for multi-photon microscopy applications. There is a need to overcome this deficiency of OPS-lasers to enable the wavelength-selection advantage thereof to be made available for optimizing the response of particular fluorophores in multi-photon microscopy applications.